feat: enhance API and session management with Nacos and Redis integration

- Add Nacos registry for service registration and deregistration.
- Implement Redis registry for session management with heartbeat and session claiming.
- Improve completion service with session handling and request validation.
- Enhance WebSocket handling for completion requests with JSON-RPC support.
- Add tests for new registry implementations and completion manager functionalities.
- Refactor existing code for better readability and maintainability.

backend/internal/cluster/nacos_registry_test.go

@@ -0,0 +1,36 @@
+package cluster
+
+import "testing"
+
+func TestParseServerAddr(t *testing.T) {
+	host, port, err := parseServerAddr("10.0.0.10:8848")
+	if err != nil {
+		t.Fatalf("unexpected err: %v", err)
+	}
+	if host != "10.0.0.10" || port != 8848 {
+		t.Fatalf("unexpected parse result %s:%d", host, port)
+	}
+
+	host, port, err = parseServerAddr("nacos.internal")
+	if err != nil {
+		t.Fatalf("unexpected err: %v", err)
+	}
+	if host != "nacos.internal" || port != 8848 {
+		t.Fatalf("unexpected parse result %s:%d", host, port)
+	}
+}
+
+func TestParseServerAddrInvalid(t *testing.T) {
+	if _, _, err := parseServerAddr("10.0.0.10:"); err == nil {
+		t.Fatalf("expected parse error")
+	}
+}
+
+func TestResolveRegisterIP(t *testing.T) {
+	if got := ResolveRegisterIP("172.16.1.9", "http://127.0.0.1:8080"); got != "172.16.1.9" {
+		t.Fatalf("expected explicit ip, got %q", got)
+	}
+	if got := ResolveRegisterIP("", "http://10.2.3.4:8080"); got != "10.2.3.4" {
+		t.Fatalf("expected host from instance url, got %q", got)
+	}
+}